CREDIT CARD AND LOAN RECEIVABLES - Credit Quality on Amortized Cost Basis (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Amortized Cost Basis of Receivables Outstanding	$ 17,361.0	$ 19,047.8
Percentage of Amortized Cost Basis of Receivables Outstanding	100.00%	100.00%
No Score
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Amortized Cost Basis of Receivables Outstanding	$ 217.5	$ 298.4
Percentage of Amortized Cost Basis of Receivables Outstanding	1.30%	1.60%
27.1% and higher
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Amortized Cost Basis of Receivables Outstanding	$ 1,580.7	$ 1,648.8
Percentage of Amortized Cost Basis of Receivables Outstanding	9.10%	8.70%
17.1% - 27.0%
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Amortized Cost Basis of Receivables Outstanding	$ 1,053.7	$ 1,108.5
Percentage of Amortized Cost Basis of Receivables Outstanding	6.10%	5.80%
12.6% - 17.0%
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Amortized Cost Basis of Receivables Outstanding	$ 1,123.5	$ 1,171.7
Percentage of Amortized Cost Basis of Receivables Outstanding	6.50%	6.20%
3.7% - 12.5%
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Amortized Cost Basis of Receivables Outstanding	$ 7,828.1	$ 8,292.1
Percentage of Amortized Cost Basis of Receivables Outstanding	45.10%	43.50%
1.9% - 3.6%
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Amortized Cost Basis of Receivables Outstanding	$ 2,944.8	$ 3,375.3
Percentage of Amortized Cost Basis of Receivables Outstanding	17.00%	17.70%
Lower than 1.9%
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Amortized Cost Basis of Receivables Outstanding	$ 2,612.7	$ 3,153.0
Percentage of Amortized Cost Basis of Receivables Outstanding	14.90%	16.50%